The Genetics of Dog Size

Sorry for the light blogging this week. I've been spending my time doing all of the things I should have done last week, but didn't because I was writing long blog entries about my experiences in Knoxville.

You might want to have a look at this brief but interesting article from today's New York Times. It describes some recent work on the genetics of dog size.

Here's how the article begins:

If it weren't for IGF-1, Paris Hilton's life would be a lot less elegant.

She'd be lugging around an Irish wolfhound in her purse.

Scientists have just discovered which gene fragment controls the size of dogs, which have the greatest size range of any mammal -- no other species produces adults with 100-fold differences, like that between a two-pound chihuahua and a 200-pound Newfoundland.

In a study to be published tomorrow in the journal Science, researchers analyzed 3,241 purebred dogs from 143 breeds. Genetically, the yapper arguing with your ankle is almost identical to the drooling behemoth bred to hunt bears, except for a tiny bit of DNA that suppresses the “insulin-like growth factor 1” gene.

Dog breeders have unwittingly been selecting for it since the last Ice Age. Dogs emerged from the wolf about 15,000 years ago, and as far back as 10,000 years ago, domesticated dogs as big as mastiffs and as small as Jack Russell terriers were trotting the earth.

Aside from the fact that I think that pretty much anything about dogs is interesting, I think there's a lesson in this for thinking about evolution. It is sometimes said that if dogs as different as a Saint Bernard on the one hand and a chihuahua on the other were known only as fossils, they would not be placed in the same species, and possibly not even in the same genus. Yet apparently this difference in size has a very simple genetic basis.

What this suggests is that differences in body structure that seem striking at the level of the phenotype could be the result of very small changes in the genes. Results like the ones reported here make it somewhat easier to accept the idea that small genetic changes can accumulate into large changes in the physical forms of organisms.